Abstract: To elucidate the mechanism by which polysaccharide hydrocolloids affect yogurt stability, the effect of adding different concentrations of carrageenan as a representative of polysaccharide hydrocolloids in yogurt on its stability was investigated along with the interaction between carrageenan and casein micelles. It was found that carrageenan at a low concentration (0.05%) significantly improved yogurt stability, but exerted the opposite effect at a high concentration (≥ 0.10%). Correspondingly, the addition of low concentrations of carrageenan resulted in more uniform protein distribution in yogurt, while high concentrations of carrageenan led to the formation of larger whey pockets. Interestingly, rheological analysis showed that adding carrageenan increased yogurt viscosity, storage modulus (G’), and loss modulus (G”), and this effect was more pronounced at higher carrageenan concentration. This finding demonstrated that carrageenan had a thickening effect on yogurt, which increased with the increase in its concentration. Analysis of the interaction between carrageenan and casein micelles found that carrageenan at 0.05% concentration and casein micelles were miscible with each other. When the carrageenan concentration exceeded 0.10%, casein micelles were aggregated and even precipitated. Additionally, the entropy change (ΔS) of mixing carrageenan solution and casein micelle suspension was negative. These results indicated that carrageenan and casein micelles were thermodynamically incompatible, and their interaction might lead to phase separation. Thus, it was speculated that at low concentrations, carrageenan was miscible with casein micelles, and played a dominant role in thickening yogurt, thereby improving its stability. However, incompatibility between carrageenan at high concentrations and casein micelles led to phase separation, thus reducing yogurt stability.

Key words: yogurt; stability; carrageenan; casein micelles; thickening effect; thermodynamic interaction